Recovery of soap fraction from recycle impregnating cooking liquor in a continuous rapid pulping system

ABSTRACT

A PROCESS FOR THE DELIGNIFICATION OF CELLULOSIC MATERIAL WHEREIN A POSTION OF IMPREGNATION LIQUIOR IS WITHDRAWN FROM AN IMPREGNATION ZONE, CENTRIFUGED TO SEPARATE THE WITHDRAWN LIQUOR INTO A SOAP LEAN FRACTION AND A SOAP RICH FRACTION AND THE SOAP LEAN FRACTION IS RECYCLED TO   THE IMPREGNATION ZONE. TALL OIL, IN THE AMOUNT OF FROM 10 TO 20 GALLONS PER TON OF PULP, IS OBTAINED FROM THE IMPREGNATION LIQUOR.

,1971 J. F. TOURTELLOTTE 3,562,096

RECOVERY OF SOAP FRACTION FRQM RECYCLE IMPREGNATING COOKING LIQUOR IN A CONTINUOUS RAPID PULPING SYSTEM Filed April 21, 1967 INVENTOR.

John F. Tourtellofle BY ATTORNEYS United States Patent RECOVERY OF SOAP FRACTION FROM RECYCLE IMPREGNATING COOKING LIQUOR IN A CON- TINUOUS RAPID PULPING SYSTEM John F. Tourtellotte, Westfield, N.J., asslgnor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Apr. 21, 1967, Ser. No. 632,578 Int. Cl. D21c 11/00 U.S. Cl. 16216 3 Claims ABSTRACT OF THE DISCLOSURE A process for the delignification of cellulosic material wherein a portion of impregnation liquor is withdrawn from an impregnation zone, centrifuged to separate the withdrawn liquor into a soap lean fraction and a soap rich fraction, and the soap lean fraction is recycled to the impregnation zone. Tall oil, in the amount of from to 20 gallons per ton of pulp, is obtained from the impregnation liquor.

This invention relates to the delignification of cellulosic material, and more particularly, relates to the recovery of valuable byproducts in wood pulping systems. Still more particularly, this invention contemplates the recycle of cooking liquor in the wood pulping systems, and the recovery of the soluble soap fraction therein.

In the rapid pulping system, cellulosic material is steamed. The cellulosic material and liquor is then introduced into the upper portion of a digester having an impregnation zone, a digestion zone, and a cooling zone. A major portion of the liquor introduced into the upper portion of the impregnation zone is withdrawn from said upper portion and is recycled. The consistency of the slurry introduced into the upper portion of the digestion zone is increased by the withdrawal of said recycle liquor. This system is more fully described in copending U.S. patent application Ser. No. 274,186, filed Apr. 19, 1963.

In conventional cellulose delignification, the black liquor produced using kraft or soda cooking liquors and combinations of these liquors, contains, among other things, the sodium salts of rosin acids and fatty acids. These salts or soaps separate out or float on top of the black liquor during evaporation to recover the alkali values thereform. Tall oil is obtained by acidification of such soluble soap fraction.

Crude floating soap is also a source of tall oil. Conventionally, in the production of tall oil from floating soap, the floating soap is split with sulfuric acid to form tall oil and brine. The tall oil is separated out and saponified with aqueous sodium hydroxide. Alternatively, purification by well-known means may be introduced at one or more stages by the process of obtaining the sodium tallate from floating soap. For example, floating soap may be purified before acidulation, or the tall oil may be purified before saponification.

Tall oil is a valuable byproduct and is used commercially in soaps and lubricants. Therefore, it is desirable to recover tall oil from the pulp cooking liquor. Furthermore, the soap fraction which accumulates during the impregnation phase in wood pulping foams during the impregnation phase and this foaming may impair absorption during said impregnation phase. Additionally, the subsequent washing of pulp may be impaired by foaming of the soap, and in some instances may result in equipment shut-down.

A primary object of this invention is to minimize the carryover of the soluble soap fraction from the impregna- 3,562,096 Patented Feb. 9, 1971 tion phase in a process for the delignification of cellulosic material.

Another object of this invention is to provide a process for the recovery of the soluble soap fraction from recycle impregnating liquor in a process for the delignification of cellulosic material.

A further object of this invention is to provide a continuous process for removing the soluble soap fraction accumulated in the impregnation phase in a process for the delignification of cellulose.

A still further object of this present invention is to provide apparatus for effecting the separation of the soluble soap fraction from recycle impregnating liquor in a liquidphase process for the delignification of cellulose.

Other objects and a fuller understanding may be had by referring to the following description taken in conjunction with the accompanying drawing in which the figure is a partial schematic flow diagram illustrating a preferred embodiment of this invention for effecting the rapid and continuous removal and recovery of the soap fraction from recycle impregnating cooking liquor in the continuous delignification of cellulose.

Briefly, in accordance with this invention, the impregnating cooking liquor stream withdrawn from the impregnation zone is passed to a multiple stage liquid-liquid centrifuge operating at the prevailing temperature and pressure of impregnation or lower, wherein the impregnating liquor is separated into a soap rich fraction and a soap lean fraction. The soap lean fraction is recycled to the impregnation zone after evaporation and concentra tion adjustment. The soap rich fraction is withdrawn from the centrifuge and passed to a settling tank. Soluble soap is skimmed off the top of the liquid in the settling tank as the desired product, and a soap lean fraction is withdrawn from the bottom of the tank and is combined with the soap lean impregnation liquor withdrawn from the centrifuge.

Referring now to the drawing, cellulosic material such as wood chips and the like in line 1 is preheated and steamed in steam zone 2, and passed through line 4 under the control of valve 14 and introduced into the upper portion of an impregnation zone 16. Recycle cooking liquor is drawn from feed tank 6 into the impregnation zone through line 12. The consistency of the chip-liquor slurry maintained in the impregnation zone 16 is from about 15 to 40%, preferably about 25%, and is determined by various factors, such as the size of the pieces of the cellulosic material, temperature and pressure of impregnation, etc.

The impregnation slurry is passed through impregnation zone 16, and all or a portion of the impregnation liquor withdrawn from the impregnation zone 16 through line 18 is passed to a centrifuge 20, which operates at temperatures and pressures similar to the impregnation zone 16, cooler 21 adjusting the temperature for proper separation. The impregnating liquor is separated into a soap lean fraction and a soap rich fraction.

The soap lean fraction is withdrawn from the centrifuge 20 through line 22 and combined with make-up liquor in line 24 and a clarified recycle impregnation liquor in line 26 as hereinafter more fully described. The mixture of liquors is heated in a heater 28 and passed through line 30 and injected into a flash tank 32. The vapor released in the flash tank 32 is withdrawn through line 34 and recycled to the steam zone 2; the excess steam being released through line 35. The concentrated impregnation liquor is withdrawn through line 8 to feed tank 6 and recycled to the impregnating zone 16 through line 12.

The soap rich fraction in the centrifuge 20 is withdrawn through line 36 and passes to cooler 38 and line 40 and passed into a settling tank 42. The cooled liquid is allowed to settle, and a light, soluble soap fraction rises to the top of the liquid in the tank. The liquid in the settling tank is skimmed from the top in the normal manner and a light, soap rich fraction is drawn off the top of said liquid and passed through line 44 and the soap is acidified to obtain the desired tall oil byproducts which are passed to subsequent units (not shown) for further processing.

The lower heavier fraction in the settling tank 42 is withdrawn from the bottom of the tank through line 26 and mixed with recycle impregnation liquor in line 22 and make-up liquor in line 24 as heretofore described.

The concentrated impregnated wood slurry is withdrawn from the impregnation zone 16 and passed through line 46 under the control of a valve 48, and introduced into a digestion zone 50, which operates in a normal manner. Liquor or direct steam at a temperature of from about 360 F. to about 400 F. is mixed with the wood chips slurry and a consistency of from about 15% to about 40%, and the slurry is maintained in the digestion Zone for a time period of from about to about 30 minutes, preferably about to minutes, to effect solubilization of the lignins in the cellulosic material to produce cellulosic fibers.

The cooked pulp is withdrawn from the digestion zone 50, and passed through line 52 under the control of a valve 54 to a washer 56, washed with water in said washer, and then withdrawn and passed through line 58 to subsequent units for further processing (not shown).

As an example of this invention, on an hourly flow basis, 3600 pounds of dry southern pine wood chips and 2400 pounds of moisture are preheated in steam zone 2 by steam at 15 p.s.i.g. The preheated wood chips were impregnated with a mixture of 6000 pounds of alkaline white liquor and 60,000 pounds of recycle liquor at a temperature of 300 F. The impregnation liquor was continuously withdrawn from the impregnation zone 16 and passed to the centrifuge 20. The specific gravity of the crude soap fraction is approximately 0.96 to 0.99, while the specific gravity of the white liquor is approximately 1.10 to 1.20.

The soluble soap formed during the impregnation step, separated and withdrawn from the system, and the remaining impregnating liquor is recycled in the manner heretofore described. Tall oil in the amount of from 10 to 20 gallons was obtained for each ton of pulp which was digested.

Utilization of the teachings in this invention results in the recovery of valuable byproducts in the continuous delignification of cellulose. Furthermore, utilization of the principles of this invention minimizes the carry-over 4 of soap fraction from the impregnation zone, and this minimizes subsequent foaming problems previously encountered in steps of digestion, pulp washing, black liquor oxidation and black liquor evaporation, normal in the delignification of cellulose.

While a preferred embodiment of my invention has been illustrated and described, variations thereof may be made by one skilled in the art, and therefore the invention as described hereinafter is intended to be limited only by the scope of the disclosure and the appended claims.

What is claimed is:

1. In a process for delignifying a cellulosic material wherein the cellulosic material and cooking liquor are introduced into an impregnation zone to impregnate the cellulosic material with the cooking liquor, the impreg nated cellulosic material is passed to a digestion zone to effect cooking thereof, and cooking liquor withdrawn from the impregnation zone is subsequently recycled to the impregnation zone, the improvement comprising: centrifuging at least a portion of the cooking liquor withdrawn from the impregnating zone to produce a first soap lean cooking liquor fraction and a soap rich cooking liquor fraction; separating soluble soap from said soap rich cooking liquor fraction to provide a second soap lean cooking liquor fraction; and recycling the first and second soap lean cooking liquor fractions to the impregnation zone.

2. The process as defined in claim 1 including recovering by said centrifuging, a soap rich cooking liquor fraction having a specific gravity within the range from about 0.96 to about 0.99 and a soap lean cooking liquor fraction having a specific gravity within the range from about 1.1 to about 1.2.

3. The process as defined in claim 2 wherein soluble soap is separated from the soap rich cooking liquor fraction by settling and skimming the soap from the settled fraction.

References Cited UNITED STATES PATENTS 6/1927 :Fish l62-41X 10/1968 Harding 16216 US. Cl. X.R. 

